Outline

Questionnaire: While the multi-factorial nature of patello-femoral (PF) instability is widely recognized, little is known about the competence of the quadriceps and its role in providing active joint stabilization in patients with PF instability. However, any imbalance between the medial and lateral compartments of the quadriceps is likely to influence the joint mechanics and could modulate the risk for PF instability. In particular, an insufficient vastus medialis (VM) is expected to result in an increased lateral pull on the patella and could thus increase the risk of lateral patellar dislocation and PF instability. This study hypothesized that an imbalance between the vastus lateralis (VL) and VM exists in patients with PF instability such that the VM is relatively smaller than the VL when compared against the conditions in healthy controls.

Methods: After approval by the local ethics committee, MRI scans of both left and right femurs of 15 healthy subjects and 13 patients with unilateral patello-femoral instability scheduled to undergo MPFL reconstruction were obtained. After 3D reconstruction of the MRI data, the quadriceps muscles were segmented and the proportions of the VL and the VM of the total CSA were quantified. A t-test was used to identify whether differences existed between the CSAs of the VL and VM between both the affected and unaffected limbs of the patients and healthy controls.

Results and conclusions: The vastus medialis CSA of the contralateral limb of the patients (1627.9Â±364.4 mm2) was significantly smaller (p<0.01) than both the VM of the patients' affected leg (1859.8Â±489.0 mm2) and the VM of the controls (2020.5Â±395.4 mm2). However, no significant differences were found for the VM between the affected limb of the patients and the controls. No differences between patients and controls were observed for the relative ratio between VL and VM. The results of this study suggest that the CSA of the VM and the VL of the affected limb in patients with PF instability is similar to the values of healthy controls. This finding suggests that in the presence of other risk factors for PF instability, a VM CSA that is within the normal range is not protective of PF instability. Unexpectedly, the contralateral limb of patients with PF instability presented a reduced VM CSA, a condition that is likely to increase the lateral force vector acting on the patella. Whether the latter resulted into functional deficits remains to be investigated. As our patients with patello-femoral instability were all subject to conservative treatment before a decision to operation was made, the lack of a difference between VM CSA on the affected limb and healthy controls might be a result of the rehabilitation protocol that specifically targets strengthening of the VM. Whether and under which conditions specific treatment of the unsymptomatic limb in patients with PFI to strengthen the VM is indicated and effective in preventing the occurrence of PFI there remains to be elucidated.